The present work is in the field of NMR apparatus and relates particularly to an NMR probe operating at cryogenic temperatures
An NMR probe coil couples an RF transmitter/receiver to sample nuclei. Operation of the RF coil at cryogenic temperatures contributes a very high efficiency through enhanced Q of the resonant circuit and significant reduction of Johnson noise. These factors provide enhanced sensitivity for NMR spectra and have motivated considerable effort to realize improved cryogenic NMR probe.
A central component within the cryogenic probe is the “cold head” (that portion within the probe structure providing thermal, and often, structural support for the RF coil). The cold head is desirably regulated at a selected operating temperature. A heat exchanger comprising a part of the cold head cools the cold head to somewhat below the operating temperature, while the application of RF power to the coil adds heat to the environment and increases the thermal load on the heat exchanger with a resulting temperature rise. Often it is desired to operate the heat exchanger for operation slightly below the desired operating temperature in order to permit temperature regulation. To avoid operating below the design temperature, a coolant heater is provided to raise the temperature of the coolant inflow to the heat exchanger at the cold head. By adjustment of heater power in this manner, regulated thermal equilibrium is achieved. Regulation of the temperature of the cold head within a narrow range is necessitated for a number of reasons. Salient of these is variation with temperature of magnetic susceptibility of materials employed in the environs of the cold head resulting in magnetic field disturbance in the neighborhood of the sample; temporal variation of temperature over lengthy data acquisition, and the stability of various parameters of the equipment. To the extent of such disturbance, the precision of NMR data is degraded. NMR is necessarily practiced in a time varying regime of RF power as applied in pulse sequences and often with substantial RF power application from a (second) decoupler coil. Thermal dependence of magnetic field inhomogeneities results in a broadening of the resonant lineshape.
Certain other considerations arise for cryogenic probes. For the purposes of this work, cryogenic temperatures refers to a range below 77 K. Thermal isolation of probe components for operation at cryogenic temperatures requires an evacuated environment. It is known that the residual gas (mainly hydrogen at cryogenic temperatures) within the probe cryostat (in combination with high voltages present at RF coils in the probe) is subject to ionization. This results in significant increase in noise and consequent reduction of the signal to noise parameter, as disclosed in the US publication US 2006/0045754A1.
It is also recognized that the mechanism of cryogenic pumping is operative within the cryostat to stabilize residual gasses by adsorption on cold surfaces. Utility of the cryo-pumping mechanism is known in prior art where the accumulation of residual gas on “cold plates” provides at least temporary stabilization and removal from the excitation/de-excitation process. This process is saturable and the cold plate pumping surfaces are allowed to warm at maintenance intervals to liberate these gasses for pumping by other pumping modalities. It is common to provide turbomolecular pumps, chemical getters or ion pumping apparatus to remove these residual gasses. The effectiveness of cryo-pumping depends upon the residual gasses and the nature of the cryo-pumping surfaces among other critical particulars. Such matters are outside the scope of the present work.
One prior art apparatus, exemplary of the above description, is schematically described in its essentials at FIG. 2a. Cold head 70 comprises a thermally regulated support structure and RF coil 71, coil former (not shown) and mounting flange 70a. The cold head 70 is cooled to (T1−ΔT) K, somewhat below a nominal desired operating temperature of T1 K (for example, 25 K) by heat exchanger 78. Details of the coupling of the RF coil 71 to the RF circuit is not shown. It should be understood that the probe cryostat housing 80 has an annular geometry defining a well or bore 82 wherein a sample is introduced at room temperature.
Coolant inflow conduit 84 provides coolant to heat exchanger 78 and warmed coolant is removed through outflow conduit 85. In order to thermally regulate the cold head, it was known in prior art to drive coolant flow at a rate sufficient to maintain the heat exchanger at a temperature slightly lower than T1K and to provide a heater 88 disposed between a portion of the inflow conduit 84 and the heat exchanger 78 to warm the inflowing coolant sufficiently to raise the operating temperature of the heat exchanger to T1. In this manner, heat developed from RF power dissipation is transported away from the cold head 70 and the operating temperature restored by heating the coolant (or alternatively, reducing the coolant flow rate) in response to an appropriate thermal sensor. One example of this prior art is the US publication US 2005/0046423A1.
In cryogenic probes of the above description the coolant inflow conduit 84 must exhibit a very high thermal impedance and stainless steel tubing is a typical choice of suitable material. The present work has detected a degree of cryo-pumping functionality at the outer surface of this inflow conduit. Heat is applied to the inflowing coolant prior to receipt of coolant by the heat exchanger 78. This acts adversely on the cryo-pumping occurring on the outer surface of conduit 84. The cryo-pumped vapors (principally hydrogen) are in some degree released with consequent deleterious effect of increased RF noise. The present work is directed to amelioration of these undesirable characteristics.